The following actions have to be taken in order to add a new router, transport,
authenticator, or lookup type to Exim:

Choose a name for the driver or lookup type that does not conflict with any
existing name; I will use “newdriver” in what follows.

Add to src/EDITME the line:

<type>_NEWDRIVER=yes

where <type> is ROUTER, TRANSPORT, AUTH, or LOOKUP. If the
code is not to be included in the binary by default, comment this line out. You
should also add any relevant comments about the driver or lookup type.

Add to src/config.h.defaults the line:

#define <type>_NEWDRIVER

Edit src/drtables.c, adding conditional code to pull in the private header
and create a table entry as is done for all the other drivers and lookup types.

Edit scripts/lookups-Makefile if this is a new lookup; there is a for-loop
near the bottom, ranging the name_mod variable over a list of all lookups.
Add your NEWDRIVER to that list.
As long as the dynamic module would be named newdriver.so, you can use the
simple form that most lookups have.

Edit Makefile in the appropriate sub-directory (src/routers,
src/transports, src/auths, or src/lookups); add a line for the new
driver or lookup type and add it to the definition of OBJ.

Create newdriver.h and newdriver.c in the appropriate sub-directory of
src.

Edit scripts/MakeLinks and add commands to link the .h and .c files
as for other drivers and lookups.

Then all you need to do is write the code! A good way to start is to make a
proforma by copying an existing module of the same type, globally changing all
occurrences of the name, and cutting out most of the code. Note that any
options you create must be listed in alphabetical order, because the tables are
searched using a binary chop procedure.

There is a README file in each of the sub-directories of src describing
the interface that is expected.